Grinding wheel structure



Aug. 2, 1960 L, COES, JR, mL 2,947,616

GRINDING WHEEL STRUCTURE Filed NOV. 12, 1957 Osc. 52

AMP.

I NVE NTO R 0E/N6 C055 Je. A44/@0N 3 CHA MPAGNE GRINDING WHEEL STRUCTURE Loring Coes, Jr., Brookfield, and Marion S. Champagne, Princeton, Mass., assignors to Norton Company, Worcester, Mass., a corporation of Massachusetts Filed Nov. 12, 1957, ser. No. 695,614 s claims. '(cl. 51--290 The invention relates to grinding wheel structure.

nited States Patent One object ofthe invention is to provide a grinding p wheel structure that is electrically conductive with an ohmic resistance suitable for certain machine control operations. Another object of the invention is to provide a process for the manufacture of `grinding wheels reliably to reproduce wheels having a desired range of electrical conductivity. Another object of the invention is to provide readily practiced processes for the manufacture of such wheels. Another object of the invention is to provide a grinding wheel of abrasive grains bonded with vitried ceramic bond` which is electrically nonconductive but having connecting porous spaces coated with an electrically conductive material readily applied in actual practice, and of relatively high ohmic resistance, whereas such a wheel without the coating of the pore spaces would be for all practical purposes a complete non-conductor. Another object is to provide a grinding wheel having an electrically conductive coating on the pore spaces of interconnected pores which is continuous, and of substantially the same ohmic resistance per cubic centimeter throughout. Another object of the invention is to provide a method of making the kind of grinding wheel structure above indicated which places on the pore space walls of the interconnecting pores of such a wheel a substantial uniform thickness coating without completely filling the pore spaces. Another object of the invention is to provide a method of coating pores of a vitrilied bonded grinding wheel with a coating which fills only a small'portion of the pores, on the order of 1%, so as to not interfere with the grinding characteristics of the wheel and which coating is electrically conductive ,within the range desired for certain grinding controlling EXAMPLE I Best mode We` procured a grinding wheel which was made of white aluminum oxide abrasive grains of a purity of better than 98%, half of it being 280 grit size and the other half (by weight) being 320 grit size, and having -a volume percentage of abrasive of 47.3%, a volume percentage of V,bondtof 15.0%, and a volume percentage of pores of 37.7%,

These pores were` interconnected. The bond formula is given inthe following table.

"ice

TABLE I Parts by weight Silica, S102 65..'13 Alumina, A1203 16.96 Iron oxide, Fe203 3.49 Iron oxide, FeO 1.72 Titanium oxide, TiOz 1.15 Zirconium oxide, ZrOZ` 0.06 Phosphorous oxide, P205 0.07 Calcium oxide, CaO 5.08 Magnesium oxide, MgO 2.73 Sodium oxide, Naz() 1.03 Potassium oxide, KZO 2.70

Thiswheel was 12" in diameter, Mi" thick and had a 3 central hole. It was made in the usual way, by coating the abrasive grains with a water solution of dextrine and then mixing therewith the bond in dry powdered form.

We soaked this wheel in heavy lubricating oil, an SAE 30 oil. This is a well known grade of automobile engine oil. The wheel was completely immersed in the oil and no vacuum or pressure was used. t It suffices to leave the wheel in the oil for afew minutes; for lassurance in this case we left it in for half an h our. The wheel took up 110 grams of the oil into its pores. The oil was well distributed throughout the pores of the entire wheel. This wheel was then put into a furnace supplied with an atmosphere of pure nitrogen. We heated the furnace to 800 C. which took about three hours. The heating rate is, however, not important. The furnace was then allowed to cool andwhen the wheel had cooled it was complete.

Figure 1 shows the condition of this wheel. The fused aluminum oxide abrasive Igrains of 280 and 320 grit size are indicated by the numeral 1,the bond is indicated by the numeral 2, the pores by the numeral 3, the coating of vitreous carbon on the pore spaces is indicated by the numeral 4, and the wheel as a whole is indicated by the numeral 5.

Vitreous carbon is a distinct allotropic form of carbon. There are now known to be four allotropic forms of carbon. The common allotropic form of carbon is graphite which has a distinct crystal structure. Another distinctly different crystal form of carbon is diamond. A third form of carbon is .amorphous carbon which probably contains some graphitic oxide first identified by Henri Mcissan, the French scientist, about half a century ago.

Amorphous carbon is formed when carbon containing gas is burned and when wood is turned to charcoal by burning it with a deficient supply of oxygen. Graphite -can be made from amorphous carbon by heating it to high temperatures in the absence of air. How nature made `the diamonds is unknown.` How man makes diamonds has not been published. It has been indicated by many scientists that there is a probable transition area from graphite to diamond at very high pressures and very high temperatures.

The above three forms of carbon have been known for many years and have been differentiated as different forms of carbon for many years. It is only recently that vitreous carbon has been differentiated -as a fourth allotropic form of carbon. It is not graphite. It is not amorphous carbon. It is probably =a transition form from graphite to diamond but probably only a short way along the road of transition from graphite to diamond. It has an X-ray pattern which is not that of graphite, is not that of diamond but is all its own and the same as that of no other substance. Amorphous carbon has no X-ray pattern.

Vitreous carbon, is formed when hydrocarbons are .Where 'above en@ .,foil the Wheel Vthe 'battwith the wheeland thecoke in a kiln and heated dissociated inthe absence of oxygen as in the Example l. This differs from the formation of amorphous carbon which takes place when hydrocarbons, carbohydrates and the like are burned withl oiygenll but with a deficiency of oxygem IVitreous carbon was apparently rst recognized asf djsitinstly, dfefsntallatropi9-f9fm 0f Carbon by Heimweh@ 56., pas@ .20711 .1.9235 Piof f6 that' if was considered to be graphite, it is not. s r v d rllhe furnaceused making the wheel of Example I is 4shown in Figure consistedl of a chamber coinprising a bottom o` and a top 7 connected by a sand seal `S with inlet pipe; 9 an outlet pipe r1`0and a bar 1v1V to hold the wheels The' inside diameter of the bottom 6? was i4 andthe overall height of the furnace waiswaisi other que@ were about 1h prof portiontheret'o as shownin figure 3. Nitrogen was pumped at the rate of three litres a minute. All parts except't the sand were Inde of stainless steel.A The furnacer was cylindrical Tlfej wheel of Examplefl had an' actual ohr'nic resistance of 100 ohms las inesuredfrom side to side of the wheel. Priorto treating wheels in accordance with Example I, ffr fourv wheels were actually treated, we piled four wheels of the same specificationsr in the furnace of Figure 3V with'a batt on the bottom of the furnace, batts la'etweenv the wheels andy a batt on top of Athe top wheel. These' baitts'were' conventional silicon carbide refractory batts and they covered the areas of the wheels. The resistance to these wheels from side to side turned out to begabout 20,000 ohms which-while' usable in accordance invention, is nowhere nea'r as satisfactory as the lower `resistance of 1'00| ohms'. Incidentally there is not muchtdilference when the actual ohmic resistance is taken from side toside' of thewheel, a distance of onequart'er of an inch, or from central hole tov periphery a distance of four and one-half inches which seems to be surprising but is undoubtedly due to the spreading of the electric current over the longer distance.

Y We attribute the improved result obtained when the wheels were free of cover'jall around to the fact that the vitreous carbonwa's deposited fromhydrocarbon in dilute vapor" phase; When the Wheel is covered, the gas surrounding the wheel is too richin hydrocarbon vapor and onlyfsoot is deposited, except for little vitreous carbon 'linden to soot everywhere. Soot is amorphous carbon. `Therefore the previous statement that vitreous carbon is formed when'hydrocarbons are dissociated in the absence `offoxygenis a generalization and the hydrocarbon in the' vapor `forni should notrbe too rich in the atmosphere. However, vitreous carbon can be depositedfrorn pure hydrocarbon vapor if the vapor' is cold while the wheel is hot. Thiscould be done by heating the wheel on one side as ja` series "o'f llames and directing a stream of pure hydrocarbon vapor at the other side and the wheel would be impregnated throughout when finally the heat had gone' through to where the hydrocarbon vapor was directed. As we cant give any better parameters than this, and as even very dilute hydrocarbon vapor in a gas iscapab'leofdepositing vitreous carbon, we shall have to `make the broad claim of depositing vitreous carbon from hydrocarbon vapor.

EXAMPLE yII r We procured exactly thesane kind of a grinding wheel descilied in 'Example I. This wheel was then ksoaked in SAE 30 il as dscribed'in Example I and it took up lthe same amount of oil intov its pores.

We lthen placed ythis wheel on a non-porous alumina batt which had an area larger than the wheel and packed petroleum coke any coke would do land also any other carbonaceous material) on it to about 1" in depth veveryi i We then placed them to 800 C. in an atmospheric of air. The coke effeetive'ly prevented the oxygen of 'the'air from reacting 41 the lubricating oil in the wheel. It is not critical how long the wheel is kept at 800 C., as long as it actually reaches 800 C. that is suicient. Actually, as soon as we were sure that the wheel had Vreached 800 C., we shut off the kiln (it was an electric kiln) and allowed it to cool which took a few hours. Then we opened the kiln and removed the batt fand wheel and* coke and picked out the wheel. T he wheel; wasy then complete anda-'had a layer of vitreous carbon 4 on the pore space walls. We have broken apart wheelsmade in accordance wit-h this-invention and nd' that layers ofvitreous icarbon are practically everywhere on' the' walls of the interconnecting pores which are allI thepores except ,for avery small percentage in wheels like those of Examples I and l1. In this example the l'ayer 4- was vitreous carbon on the bottom covered with soot. The resistance of the wheels, measured in the same way, was about 40,000 ohms but this wheel is usable for the purpose described. herein.

The invention is not at all limited to the kind-of wheel which `is lined withvitreous carbonl as to the abrasive thereof or the sizel thereof or the porosity thereof providedit has interconnectingy pores so that an electric current can *be passed through. it through the vitreous carbon but the bond of the wheel, although it mayrvary widely in composition is vitrilied ceramic bond, of which Table yI is an example. The following are additional examples of how to impregnate the pores of vit-rilied bonded grinding wheels with a coating of viueous carbon.

EXAMPLE Heat the' wheelr in a furnace held between 700?l C. and 1,000o C. in an atmosphere of an inert gas such as nitrogen, hydrogen, argon, krypton, helium, xenon containing hydrocarbonV vapor. In this* process nitrogen and hydrogen are inert and the others mentioned are y'the truly inert gases. This can be done as follows: Put-the wheel in the furnace, put also a quantity of gasoline `in a sagar lin the furnace say equal in weight to the weight of the wheel, and then raise the temperature of the furnace to 800 C. The furnace used to make wheels according toY this example can be the furnace of Figure 3 with the inert gas flowing at the same rate, `out it must be understood that such specifications are merely illustrative in all of the examples, as many different types of furnaces 'and'kilns can be used and the lrate of flow of the inert gas can vary widely. Wheels treated in accordance with this example have vvan ohmic 'resistance'of about 1100 ohms, raising the .temperature to 800 C. in three hours, the wheels being l2 x 1A x 3`wh`eels. The vv'vheels should be uncovered in the fur'nace as in Example -I.

EXAMPLE IV Heat the wheel in any furnace having an atmosphere of carbon monoxide to a temperature above 500 C., 1,000 C. is recommended. At 'temperatures 'above 1,300u C. many vitried bonded grinding 'wheels would be damaged but in the case of some wheels even this temperature could be used. The carbon monoxide dissociates into carbon dioxide and carbon and the carbon is deposited in the pores ofthe wheel' as Ia layer Vin all of the interconnecting pores `of vitreous carbon. Inthis example the carbon monoxide is kept flowingthrough the furnace during Ythe entire process after the wheel reaches some minimum temperature which typically is 500 C. Of course the carbon dioxide with some of the carbon monoxide ows out of the furnace and this keeps thelpercentage of carbon dioxide low yandthe ,percentage of carbon monoxide high. As -a more detailed specification of Vthis example, 'use the furnace of Figure v3 with the 'carbon monoxide 'flowing at 3 litres a minute and raise the Ytemperature to 1000 C. in four hours.

Wheels'of 'thesaine Ysize,v'1r2"'x %""x 3" have a resistance "of about 20,000 ohms. kThe wheels 'should beuncovered in the furnace.

` Wheels according to the invention are useful for controlling grinding operations as illustrated in the diagram of `Figure 2. In Figure 2 the work piece 12is being ground by a grinding wheel 5 made inaccordance with the invention. In Example Ithe manufacture of a cylindrical disc `wheel was described, but this can be trued to a shape like that shown in Figure 2.and this is the Way to make such a shape. 1`The wheel 5 is mounted on a spindle 13 which is rotated' as by means of belts 14. AsFigure 2 is simply la, diagram,` a feed nut 15 is represented which is connected to a slide 16 to move the spindle 13 forward and' back in the directionof the work, thespindle 13 being journalled on the slide `13. The nut is moved by ascrew shaft 17 driven by an electric motor 18. Referring now touthe lower right of Figure 2 and working to the leftand up, thearmature of the motor 18 `is energized by power lines 20 through a double relay switch 21, another double relay switch 22, lines 23, a slide controlled switch 24 and lines 25. The slide controlled switch 24 has an insulated element attached to the slide itself which,` when it with- -draws to `a certain point, opens the circuit as diagrammatically indicated. t 't Referring now to the upper middle right of Figure 2 and working to the left and up; the' stopping and starting of the motor 18 is controlled by lines 28 through an off and on control 29 energized by anttamplifier 30 which receives itssignal Afrom acapacitance bridge 31 energized by an oscillator 32.-` The capacitance b ridge 31 is connected bywires 34 `and` 35 to condenser plates 36 and 37. Theother elements of the condensersof which the plates 136 and 37 are parts are the faces 40 and 41 of the wheel 5. When the capacity of the circuit of the bridge 31, wire 34, plate 36, face 40, wheelV 5, face 41, plate 37 and wire 35 rises,` toa certain high value during the feeding of the wheel 5 into the work piece 12, `the signal from the bridge 31 through the amplifier 30 to the control 29 opens` the switch 22 by means of relay solenoid 42"which` stops the infeed. But when the wheel S has been worn away some or trued, thus becoming of less diameter, the capacitances between 36 andA 40 and 37 and 41 `are lowered and the wheel is fed again toward the work piece to` maintain the same depth of cut. The arrangement shown` in Figure 2 is particularly useful for thread grinding of 4all kinds and can also be used for surface grinding and cylindrical traverse grinding. Since this invention is in a grinding wheel structure and a method of producing it we dont need completely to describe the machine, since the above description is merely to show the utility of the wheel and such machines actually exist.

However the machine should have a circuit to cause the slide 24 to withdraw at the end of a grinding operation. Referring to the lower right of Figure 2, a push button switch 45 is connected by lines 46 to the power lines and when closed, through lines 47 energizes a relay solenoid 48 which closes a double relay switch 49 and opens the double relay switch 21. The double relay switch 49 is connected by lines 50 to the power lines 20 and by lines 51 to the lines 23, and it will be seen that the direction of the current is reversed through the double relay switch 49 as compared with through the double relay switch 21. The motor 18 is a reversible motor since the current through its field coils is not reversed as they are connected by lines 20a to the lines 20 not through the switch 49, so when the push button switch 45 is closed the slide 16 retreats, and when it gets to a certain position the switch 24 opens the circuit andthe motor 18 stops thus stopping the withdrawal of the slide. Referring to the bottom middle left of Figure 2, to start the machine up again the operator mot mentarily presses a push button switch 55 which connects .lines 56 to lines 57, the former being connected to lines 23 and the latter to lines 25. Later the circuit is reestablished through the slide switch 24. Y

between the faces 40 and 41 will be about the same asthat from side face to side face of the wheel.

The lining of vitreous carbon should not be too thin, or the ohmic resistance will be too great for the best resultsin controlling the ymachine as described. The lining of vitreous carbon should not `be too thick or the grinding performance of the wheel will suier. For the best results the lining everywhere in the electrical path should'be not less than .01 mil thick and not more than 10 mils thick but usable wheels can have linings outside of these-limits as our wheels can be used in other machines.

Any wheel that has interconnecting pore spaces will also have surface pores. When the interconnecting pore spaces are coated with vitreous carbon, so also will be coated the surface pores.

Preferred grinding wheels according to the invention will have an ohmic resistance of less than 500 ohms from one surface to another surface, such as from the surface 40 to the surface 41. But in the case of use of the wheels for cylindrical grinding, the two surfaces will be two parts of the same surface, the cylindrical peripheral surface; however a wheel so used will have two opposite side surfaces between which the resistance can be measured. There is` no lower limit to the ohmic resistance for use of the wheel as described, as from surface to surface it could have a resistance of zero ohms if such were possible and the condensers 36-40, 37-41 would still operate properly, in fact they would be more sensitive.

What is vitried ceramic bond is very well understood. It excludes all forms of organic bond and all metal bonds and metal carbide bonds. invariably on analysis it shows a major portion of oxides, a major portion of the wheel being metal oxide, silica being taken to be metal oxide. In the usual case it consists of a complex silicate. Vitried means that the bond has been fused to flow around the grains thus to bond them together.

Although the examples show the use of aluminum oxide abrasive any abrasive can be used. Aluminum oxide abrasive is usually preferred for grinding steel. Next in order by amount of use cornes` silicon carbide and `next after that comes diamond. These three abrasives are used for the manufacture of well over 99% of all grind ing wheels made today, but Izirconia abrasive has been tried and found useful for certain grinding operations, so also have mixtures of lzirconia and titania. Various other oxides and also the carbides and the borides of the transition metals of groups IV, V, and VI can be used. These transition metals are Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W. All kinds of mixtures can be used. The invention is therefore not limited to the use of any particular abrasive.

Typical examples of inert gases have been given. In this invention any gas is inert that is inert to hydrocarbon vapor and which does not itself dissociate at the top temperature used. The gases mentioned are all elemental gases so they cant dissociate and all of them are inert to hydrocarbon vapor at any temperature.

In Figure 1 the pores appear to be blocked but this is because Figure l represents a cross section and in any cross section the pores will appear to be blocked. The connections are through .the third dimension, but not necessarily at right angles to the plane of the paper.

With regard to the firing temperature to impregnate the wheels with vitreous carbon, temperatures down to 450 C. can be used with some results and may be preferred where expense is the major consideration. Although most grinding wheels are fired at temperatures which do not much, if any, exceed 1300 C. a vitried ceramic bonded grinding ywheel could be made out of a composition to be fired at cone 35 involving a top temperature of 17803" C. Just use the aluminum oxide bond mixture now .usedforf-making highly refractory furnace bricks'and's'hape itinto a. Wheel with a.- central hole and fireit at cene-35. It will. grind although itwill not grind most steels very well, -but therefore we claim up to 1780" C. as the tiring temperature for impregnating the wheel because any wheel can be ytreated according to this invention at anyr temperature up to the tiring. temperature at whichJitwasmatured. v

The 'only' utility We know of for=electrically conductive grinding Wheels is for use :in machine tools. .Nothing but vitried ceramic'.bonded` grinding, wheels are used in machine tools, except in the case `of diamond wheels. But probably the only machine tools which will use electrically conductive wheels of ohmic resistance over 50 are precision grinders and they use practically nothing but vitrifred ceramic bonded wheels. Also metal bonded wheels are highly conducting and are only suitable for a limited number of grinding operations. Organic bonded grindingy Wheels are practically never used on precision grinders and they would be destroyed at l450 C. anyway. So our invention must be embodied in orin the manufacture of a vitrfied ceramic bonded grinding Wheel.

Alt will thus be seen that. there has been provided by this invention a grinding Wheel structure and a method vofrnaking it in accordance with which the various objects hereinabove'set -forth together with many thoroughly practical advantages aresuccessfully achieved. As many possibleen'rbodiments may be made of the above invention and as many changes might be made `in the embodiments above set forth, it is -to be understood that all matter hereinbefore set forth or lshown in the accompartying drawings is to be interpreted as illustrative and not in a limiting sense.

We claim:

Xl. A grinding wheel consisting of abrasive grains bonded together by Yelectrically non-conductive vitriied ceramic bond leaving interconnecting ,pore spaces in the wheel, and a continuous lining of vitreous carbon on the walls of the interconnecting pore spaces said vitreous carbon being electrically conductive and said lining forminga continuous electrical path through the wheel, said wheel having surfaces and said path extending from one surface to another surface `and said continuous lining being.alspfenI the walls of the surfacefporesftoffsad Suf'feef; v ai y 2.,Agrindingj -wheel Vaccording Ytof-claim 1; said -wheel having V@resistance oflessthan 500-ohms from one-:0f said surfaces ito the `other'thereof.f i. ,7. f1 x Y.. 3. A grinding wheel according to claim 2, said lining being everywhere inthe path not irl/ess than .01 mil thick and not more thanrlmils thick. f f 4.C A grinding wheel according to claim 1', said lining being everywhere in the. path not less than .01 mil thick and not. more than .1() mils thick. 5.-=,Method;of making .electrically conductive a vitried ceramic bonded. grinding vwheel Shaving interconnecting pore spaces and surface pores. which consists inheating thefwheel lto a. temperature of between 45.0 and .17809 C. in an oxygen free atmosphere ofghydrocarbon vapor, dissociating the hydrocarbonvapor and depositing carbon in'- a rcontinuous liningof vitreous carbon in said interconnecting poret-spacesiand surface pores and'forming a :continuous electrically conductive path through the wheel fromA onesurface thereof to another., v6. Method according to .claim 5 jinwhich the atmosphere is also an atmosphere of a gas that is inert tohydrocarbon lvapor and whiclfrdoesv not itself dissociate at the top temperature used.V y, .7. Method according to claim 6 in which thehydrlocarbon vapor is vproduced by first filling the porespaces of the Vwheel4 with liquid hydrocarbon-and thereafter Y heating the wheel uncovered. inthe atmospherethereby generating the hydrocarbon atmosphere hut allowing `it to become dilutedv by the-inert gas by .escape 'of the hydrocarbon vapor from the pore spaces fand/entry -of the inert Agas therein. v y

8. YMethod according to claim 7 in which the inert gas `is keptowing adjacent the wheel to keep out oxygen 1,996,851 Benner .et al. v Apr. 9,

" 2,052,194 SandOrl Aug. 25, '193.6 2,342,121 Cieli Feb. 22, 1944 2,534,129 HQWe Dec. 12, 1950 2,736,642 vBaked' et al. Feb. 28, .1956 

1. A GRINDING WHEEL CONSISTING OF ABRASIVE GRAINS BONDED TOGETHER BY ELECTRICALLY NON-CONDUCTIVE VITRIFIED CERAMIC BOND LEAVING INTERCONNECTING PORE SPACES IN THE WHEEL, AND A CONTINUOUS LINING OF VITREOUS CARBON ON THE WALLS OF THE INTERCONNECTING PORE SPACES SAID VITREOUS CARBON BEING ELECTRICALLY CONDUCTIVE AND SAID LINING FORMING A CONTINUOUS ELECTRICAL PATH THROUGH THE WHEEL, SAID WHEEL HAVING SURFACES AND SAID PATH EXTENDING FROM ONE SURFACE TO ANOTHER SURFACE AND SAID CONTINUOUS LINING BEING ALSO ON THE WALLS OF THE SURFACE PORES OF SAID SURFACES. 